The present invention relates to a method to minimize corrosion and particularly build-up in sections, including associated feed conduits, of a flue-gas system where significant amounts of moisture and/or sulfuric acid are present.
In most flue-gas systems, for safety and environmental reasons, as a means of conserving heat, the flue-gas leaving the furnace at relatively high temperatures is passed through a variety of treatment devices before escaping into the atmosphere. Among these devices are, usually in sequence, a boiler or heater, a precipitator, a gas/gas heater, and a scrubber, the flue-gas returning to the gas/gas heater on its way to the stack. The temperature of the flue-gas decreases as the gas passes through the system, and in the course of that temperature decrease moisture, as water and often as sulfuric acid, comes into being. It has long been customary to add substances to the flue-gas to minimize or prevent corrosion of the exposed surfaces of the system. (My prior U.S. Pat. No. 4,842,617 of Jun. 27, 1989 entitled xe2x80x9cCombustion Control By Addition of Magnesium Compounds of Particular Particle Sizesxe2x80x9d, and U.S. Pat. No. 5,034,114 of Jul. 23, 1991 entitled xe2x80x9cAcid Neutralizing Composition Additive With Detergent Builderxe2x80x9d are representative of the use of such additives.) The corrosive action of sulfuric acid on exposed surfaces of the system is obviously undesirable and it is therefore common to add such substances as limestone or magnesium oxide to the system to neutralize the sulfuric acid. Because a solid/liquid reaction rate is generally slow, relatively large amounts of such additives must be provided. They are usually pneumatically injected into the affected portion of the system through conduits, usually in the form of pipes, using pressurized air as the vehicle to transport the additives through the conduit to the injection location in the system. The act of compressing air generates both heat and moisture, and hence the pressurized air which does the conveying is usually both moisture-laden and hot. Movement of the pressurized additive through the conduits results in some condensation of the moisture on the conduit surface and this enhances the tendency of the solid additives to stick to and build-up on those surfaces. As a result it is periodically necessary to take the injection equipment off line for cleaning, a process which is itself costly and time consuming, and while the injection equipment is off line no anti-corrosion additive is fed to the system, thus increasing the likelihood of corrosion.
When the system is provided with a scrubber the flue-gas emanating from the scrubber has a comparatively high moisture content and a comparatively low temperature, thus leading to the condensation of comparatively large volumes of moisture, significantly including sulfuric acid in its liquid form because its temperature is below its dew point. When, as is usually the case, the output from the scrubber is fed back to the gas/gas heater the moisture content of the flue-gas becomes a significant corrosion-producing factor.
I have discovered that the build-up of additives such as, typically, limestone or magnesium oxide in the conduits conveying those additives to the system can be significantly reduced and the anti-corrosion effect of the limestone, magnesium oxide or other anti-corrosion additives can be enhanced, by including with the additives, particularly as they are conveyed through their conduit and enter the system, and also importantly while the additives are in the gas/gas heater, relatively small amounts of a generally inert bulking agent in expanded form. Expanded vermiculite and expanded perlite are representative of such substances, which exhibit a crystal structural change to a xe2x80x9cpopcornxe2x80x9d type expanded material when heated to elevated temperatures, usually of 800xc2x0 F. or higher, and retaining that expanded characteristic after the high temperature has been reduced. The expansion is normally on the order of 2 to 5 times the original volume.
The precise mechanism by which these expanded materials perform their good offices when thus used in flue-gas systems is not known for certain, but is believed that it is because they may be able to absorb within their interstices substantial quantities of the moisture which is present without congealing or settling out.
Moisture appears to be a factor in forming accumulations of the additive on affected surfaces of a flue-gas system and in particular on the surfaces of the additive feed conduits, and the reduction in the amount of available moisture when the method of the present invention is carried out appears to be responsible for a significant lessening of the conduit build-up, as well as a lessening of corrosion throughout the treated portions of the system.